"More than a billion" is enough to add the -s in English grammar.
The poster you replied to wrote "so yeah probably less than two billions combine", so that actually means you could not use the "-s" ending in this case.
Which is all rather not the point because that's not what I said. I said "Billions have been lifted out of poverty." The meaning of that statement is: "the count of people who have been lifted out of poverty is measured with the 'billion' unit." That even includes 1 billion, exactly. If I had said "it numbers in the thousands", then 1,000 would have been an acceptable value, as would 9,9999.
But see my other comment. The number of people in India and China who have been lifted out of poverty in the last ~50 years or so is much more than a billion.
To be pedantic, “billions” is used without a specific number as a general measure of broad scale, but with any specific number, it is just “billion”, not “billions”. “billions of dollars” is fine, but “1.0000000001 billion” or “2 billion” or “999.999999999 billion”; none of the last three take an “s” at the end.